1. Field of the Invention
This invention relates to techniques for manufacturing substrate-based electronics such as integrated circuits, Liquid Crystal Display panels, and Printed Circuit boards. In particular, this invention relates to techniques for minimizing the negative effects of field curvature during the exposure of a substrate in the lithography portion of the manufacturing process.
2. Description of the Prior Art
In accordance with conventional manufacturing techniques, substrates are coated with a thin layer of an ultraviolet light-sensitive chemical, called photoresist, or more commonly, simply resist. The substrate material varies depending upon what electronic component is being manufactured. In the case of integrated circuit manufacture, the substrates are thin disks, called substrates, usually of silicon, on the order of 100 to 200 mm in diameter. In the case of manufacturing Printed Circuit Boards, referred to as PCBs, the substrate material and size vary widely, but typically are some plastic material. In the case of manufacturing Liquid Crystal Display panels, referred to as LCD panels, the substrates are typically a glass material, but many other materials could be used. During the exposure step in the manufacturing process, the coated substrates are mounted on a platform called a chuck, and typically held in place by vacuum applied from below.
The chuck is usually mounted on a swiveling or pivoting platform called a substrate stage, or simply stage, which is used to move the chuck, and therefore the substrate mounted thereon, both up and down, front to back and side to side. In recently developed lithography exposure tools, the stage can be rotated to tip and tilt the substrate from side to side and front to back at each exposure location.
During exposure of the substrate, imaging radiation, usually ultraviolet light, is passed through a matrix of opaque and transparent regions, called a mask, which includes an image to be applied to the substrate. An imaging system, usually an optical projection lens between the mask and the resist-coated substrate, is used to focus the image of the mask onto some portion of the resist-coated substrate. This or any region of the substrate which is exposed with a single image of the mask is known as an exposure field. In the process of step-and-repeat lithography, after exposure of one exposure field, the stage is moved and a portion of the resist-coated substrate adjacent to the previous exposure field is then exposed. This process is repeated until an array of exposures substantially covers the surface of the substrate. In accordance with conventional techniques, the resist is developed after exposure to leave a resist pattern on the substrate. The substrate is then processed by etching, ion beam radiation, diffusion, or some other technique to form the desired pattern having some desired characteristics. Then the resist is removed, and the entire process may be repeated with a different mask to form some other pattern on the substrate. This process may be repeated many times until all desired circuit layers have been formed in the substrate. In the case of integrated circuit manufacture, the individual circuit patterns contained in the exposure fields in the substrates are then physically separated to form a series of identical semiconductor chips.
As the circuit element density required in the exposure of substrates, in particular integrated circuit chips, has increased, and the resulting minimum linewidths being exposed has correspondingly decreased, it has become more and more difficult to properly focus the mask image on and in the resist to provide a high resolution image. The projection lens typically used to apply the image to the resist has been developed to include 10 to 25 or more individual lenses or lens elements arranged in a group in an effort to provide maximum resolution. However, the inability to further improve image resolution is still a limiting factor in increasing the density of exposed images in the PCBs, LCD panels, and in particular, the integrated circuits now being manufactured.
One of the major remaining factors limiting further improvement in image resolution on PCBs, LCD panels, and integrated circuit substrates, is field curvature. In conventional manufacturing techniques, the layer of resist in which the image is to be formed is very thin and flat, typically on the order of 1.0 to 1.5 microns thick. During exposure of the mask image onto the resist layer, the best image position, or region of best focus, is targeted to be approximately halfway through the thickness of the resist layer. It has been the general goal of conventional lithography exposure tools to attempt to maintain the resist layer as flat as possible during exposure. This is because an effectively flat image field has been the design goal of the designers and manufacturers of the projection lenses utilized. However, due to the physics of the imaging process, the lens designers must attempt to balance other categories of image-degrading aberrations against field curvature. The result is that although conventional lens designs attempt to maintain an effectively flat exposure field over the region being exposed, some amount of field curvature is usually present. The lens manufacturing processes often add additional undesired field curvature.
Substrate deforming techniques have been developed in an attempt to reduce the effects of field curvature on resolution. Such conventional techniques typically provide a series of vertically adjustable points used to deform subsets of the substrate surface. Such techniques may use stage movement to help minimize the effects of field curvature on resolution.
As shown for example in U.S. Pat. Nos. 4,666,291, 4,788,577, and No. 5,184,176, some conventional substrate-deforming mechanisms and procedures require vertical movement of at least a subset of surfaces built in the chuck. These techniques can only simulate a spherical contact surface between the chuck and the substrate, in part because each point of contact with the substrate resulting from a vertical adjustment maintains it original slope. As a result, most if not all such points of contact between the chuck and the substrate are not part of a single spherical surface.
Another conventional approach is shown, for example, in U.S. Pat. No. 4,688,932, in which a particular pattern of movement of the stage during exposure is used to attempt to compensate for field curvature.
What are needed are techniques for improving the effective resolution of the mask images applied to the resist during the lithographic exposure process.